July / August, 2004
PROGRESS: Digester is Center of Michigan Manure System
Integrated manure management systems
provide new opportunities for dairy producers.
Integrated manure management systems that reduce
nutrients and possibly eliminate pathogens in the manure
stream will change the way dairy farmers handle manure on
the farm. What was once a relatively simple task of hauling
and spreading manure on farm fields is evolving into a highly
sophisticated process involving treatment and other
operations that grant farmers flexibility and generate the
potential for new profit centers.
Michigan State University researchers in the Department of
Biosystems & Agricultural Engineering have been at the forefront of developing manure
management systems in this country. The first component they developed for dairy farms was a
sand-manure separator that took sand used for bedding out of the manure. Removing the sand
allowed more flexibility in manure handling. And because the separator also rinses the sand,
farmers can reuse it.
“Michigan dairy farmers have identified manure management as one of the most important
challenges they face today and expect to face in the future,” says William Bickert, professor of
Biosystems & Agricultural Engineering. “We are continuing to look at new ways to help farmers
meet these challenges.”
A component of the integrated manure management system being investigated by MSU
researchers is a fixed-film anaerobic digester that is used to convert the biomass into an energy
source. The fixed-film digester has the advantage of a shorter retention time -- four days
compared with 21 or more days in a plug flow system -- requiring a substantially smaller digester
volume, so the digester can be built for less cost.
“Unlike the plug flow systems operating in Michigan 20 years ago, the fixed-film digesters offer
new opportunities for treating animal manure, especially manure streams with higher moisture
contents,” Bickert says. “When used in conjunction with other manure treatment processes, the
digester reduces odors and pathogens, generates energy and sets the stage for removing
nutrients from the manure stream.”
In spring 2004, Bickert and his colleagues in the MSU Department of Biosystems & Agricultural
Engineering will install the first anaerobic digester on a Michigan dairy farm in 20 years.
Funding for the digester comes from the Animal Agriculture Initiative and the Michigan Biomass
Energy Program. In addition, Wirth and Fedewa Construction and MPC Cashway Lumber have
provided a building for laboratory equipment and instrumentation. Green Meadow Farms is
providing facilities and assistance with construction.
The digester will be one component of a complete integrated manure management system
installed on the mid-Michigan dairy farm. If the trial is successful, researchers expect to see more
digesters in place around the state.
“Anaerobic digestion is the cornerstone of an integrated manure management system in which
successive treatments may lead to a zero effluent discharge manure system,” Bickert states. “For
example, phosphorus separation technologies and other innovative manure treatments benefit
from the stable and consistent effluent stream produced by an anaerobic digester.”
The anaerobic digester also has the capability of destroying pathogens such as E. coli and
Salmonella that may be present in the manure.
How the system works:
The first step in the process is the removal of sand and grit from the manure. The digesters used
in the past were unable to handle manure with sand in it. The development of the sand-manure
separator makes the digester a viable concept again.“We have used some new science along
with age-old technology to develop a system that we hope will benefit today’s dairy farmers,”
Bickert says. “We are going well beyond generating methane.”
From the sand separator, the manure stream travels on through a grit removal system that
removes fine sand and grit particles, further reducing problems in the digester. Once the grit is
removed, the manure stream goes through a grinder, then through a heat exchanger and then to
the anaerobic digester.
“Through this process, all the biosolids in the manure will be included in the digestion process,
increasing biogas production and reducing the stream of unprocessed biosolids,” Bickert
The liquid and biosolids leaving the digester will then pass through a struvite reactor, where the
phosphorus is removed in a sludge that can then be land applied or composted. The sludge and
resulting compost can be sold as a fertilizer. (The struvite reactor is being constructed with a
grant from the National Center for Manure and Wastewater Management.)
The liquid leaving the struvite reactor, with very low concentrations of biosolids and nutrients, will
then be directed to a constructed wetland.
“The wetland effluent may be used for irrigating crops, for flushing in the barn or for other
purposes, if disinfected,” Bickert says.
Evaluating the results:
A team of researchers from the departments of Biosystems & Agricultural Engineering and Civil
and Environmental Engineering will evaluate the integrated manure management system at each
step in the process. Samples from each process will be collected and tested to determine the
effectiveness of the complete system.
Jim Wallace, a graduate student in biosystems engineering, is studying the use of an anaerobic
membrane as a part of the system to determine the impact on digester efficiency and on
reduction of pathogens.
Already the researchers have conducted hundreds of experiments in the lab, evaluating various
manure characteristics. These research trials will be beneficial when designing individual farm
“Integrated manure management systems can be tailored to individual farms, depending on the
needs and goals of the operation,” says Dana Kirk, graduate assistant in biosystems engineering.
“Individual farm operations may not need every possible treatment component but may
implement those that are useful on their farm.”
Contact: Laura Moser